Mobile station assisted soft handoff in a CDMA cellular communications system

Abstract

In a code division multiple access (CDMA) spread spectrum cellular communication system in which a mobile station user communicates with another system user via at least one base station, wherein each base station transmits a common pilot signal of a different code phase with respect to other base stations in said system, a method for directing communications between said mobile station user and said base stations. The mobile station monitors the signal strength of pilots and reports the measured signal strength to a system controller via the base station through which it is communicating. Command messages from the system controller to a new base station and the mobiles station establishes communication through the new base station in addition to the communication through the current base station. When the mobile station detects the falling below a predetermined level of the signal strength of a pilot corresponding to at least one of the base stations through which the mobile station is communicating, the mobile station reports the measured signal strength indicative of the corresponding base station to the system controller via the base stations through which it is communicating. Command messages from the system controller to the identified base station and the mobiles station terminates communication through the corresponding base station while communications through the other base station or base stations continue.

We claim: 
     
       1. In a code division multiple access (CDMA) spread spectrum cellular communication system in which a mobile station user communicates with another system user via at least one base station, wherein each base station transmits a common pilot signal of a different code phase with respect to other base stations in said system, a method for directing communications between said mobile station user and said base stations comprising the steps of: measuring at a mobile station signal strength of base station transmitted pilot signals;   communicating a signal strength message, from said mobile station via at least one base station said mobile station is in communication with, when at least one measured pilot signal each corresponding to a base station to which said mobile station is not currently in communication with, exceeds a first predetermined level;   communicating, from at least one of said base stations said mobile station is in communication with and each base station corresponding to said mobile station measured pilot signal exceeding said first predetermined level, a direction message; and   establishing, at said mobile station in response to said direction message, communications through each base station corresponding to said mobile station measured pilot signal exceeding said first predetermined level.   
     
     
       2. The method of claim 1 further comprising the steps of: communicating another signal strength message, from said mobile station via at least one base station said mobile station is in communication with, when at least one measured pilot signal each corresponding to a base station to which said mobile station is currently in communication with, falls below a second predetermined level;   communicating, from at least one of said base stations said mobile station is in communication with, another direction message; and   terminating, at said mobile station in response to said another direction message, communications through each base station corresponding to said mobile station measured pilot signal fallen below said second predetermined level.   
     
     
       3. The method of claim 1 further comprising the step of generating said direction message identifying base stations, whose pilot signals have previously exceeded said first predetermined level, to which said mobile station is to communicate through. 
     
     
       4. The method of claim 2 further comprising the step of generating said another direction message identifying base stations, exclusive of those base stations whose mobile station measured pilot signals have fallen below said second predetermined level, to which said mobile station is to remain communicating through. 
     
     
       5. The method of claim 3 further comprising the step of generating said another direction message identifying base stations, exclusive of those base stations whose mobile station measured pilot signals have fallen below said second predetermined level, to which said mobile station is to remain communicating through. 
     
     
       6. In a spread spectrum cellular communication system in which a mobile station user communicates with another system user via at least one base station, wherein each base station transmits an identifying pilot signal of a different code phase with respect to other base stations in said system, a method for directing communications between said mobile station user and said base stations by a system controller comprising the steps of: providing to said mobile station a first list identifying a first set of base stations;   measuring at said mobile station signal strength of said pilot signals transmitted by of each base station of said first set of base stations;   comparing said measured pilot signal strength to a first predetermined level;   generating at said mobile station a second list identifying a second set of base stations by removing from said first list a base station listing having a pilot signal strength measurement of a corresponding base station which exceeds said first predetermined level and placing said base station listing in said second list;   providing by said mobile station said second list to said system controller;   providing to said system controller from said base station identified in said second list availability information about resources at each of said base stations identified in said second list;   generating at said system controller a third list identifying a third set of base stations based on said first list, said second list and said base station availability information;   providing to said mobile station by said system controller said third list;   removing at said mobile station each base station listing of said third set of base stations from said first list and said second list; and   communicating by said mobile station through said third set of base stations.   
     
     
       7. The method of claim 6 further comprising the steps of: measuring at said mobile station signal strength of said pilot signals transmitted by each base station of said second and third sets of base stations; and   providing by said mobile station a particular base station listing of said second list to said system controller, wherein a pilot signal strength measurement of a particular base station corresponding to said particular base station listing exceeds a pilot signal strength measurement of a base station corresponding to a base station listing of said third list by a predetermined amount.   
     
     
       8. The method of claim 7 further comprising the steps of: providing to said system controller from said particular base station information about an availability of resources at said particular base station;   generating at said system controller an updated third list comprising said particular base station listing;   providing to said mobile station by said system controller said updated third list;   removing said particular base station listing from said second list; and   establishing communication by said mobile station with said particular base station.   
     
     
       9. The method of claim 7 further comprising the steps of: comparing said pilot signal strength measurements for said third set of base stations to a second predetermined level;   providing by said mobile station a second particular base station listing of said third list to said system controller wherein a pilot signal strength measurement of a second particular base station corresponding to said second particular base station listing is less than said second predetermined level for at least a predetermined time interval;   generating at said system controller an updated third list by removing said second particular base station listing from said third list;   providing to said mobile station said updated third list; and   discontinuing communication by said mobile station through said second particular base station.   
     
     
       10. The method of claim 7 further comprising the steps of: providing to said system controller from said particular base station information about an unavailability of resources at said particular base station; and   maintaining at said system controller said third list unchanged.   
     
     
       11. The method of claim 7 further comprising the steps of: generating at said mobile station a fourth list identifying a fourth set of base stations, said fourth list identifying all base stations unlisted in said first, second, or third lists;   measuring at said mobile station signal strength of said pilot signals transmitted by each base station of said fourth set base stations;   comparing said signal strength measurements of said fourth set of base stations to said first predetermined level; and   removing a second particular base station listing from said fourth list and placing said second particular base station listing in said second list if said signal strength measurement of a second particular base corresponding to said second particular base station listing exceeds said first predetermined level.   
     
     
       12. The method of claim 11 further comprising the steps of: identifying by said mobile station said second particular base station to said system controller;   providing to said system controller from said second particular base station information about an availability of resources at said second particular base station;   generating at said system controller an updated third list comprising said second particular base station listing;   providing to said mobile station from said system controller said updated third list;   removing said second particular base station listing from said second list; and   establishing communication by said mobile station with said second particular base station.   
     
     
       13. The method of claim 11 further comprising at said mobile station the step of, for each base station listing that has been in said first list for more than a predetermined time interval, removing each such base station listing from said first list and placing each such base station listing in said fourth list. 
     
     
       14. The method of claim 7 further comprising at said mobile station the step of, for each base station having a corresponding signal strength measurement less than a second predetermined level for at least a predetermined time interval, removing each corresponding base station listing from said second list and placing each such base station listing in said first list. 
     
     
       15. The method of claim 7 further comprising the steps of: generating at said system controller a next third list identifying a next third set of base stations;   providing to said mobile station said next third list;   comparing said next third list to said third list to identify a second particular base station listing included only on said third list; and   placing said second particular base station listing in said second list if a pilot signal strength measurement of a second particular base station corresponding to said second particular base station listing has exceeded a second predetermined level for a predetermined time interval.   
     
     
       16. The method of claim 7 further comprising the steps of: generating at said system controller a next third list identifying a next third set of base stations;   providing to said mobile station said next third list;   comparing said next third list to said third list to identify a second particular base station listing included only on said third list; and   placing said second particular base station listing in said first list if a signal strength measurement of a second particular base station corresponding to said second particular base station listing is less than a second predetermined level for a predetermined time interval.   
     
     
       17. The method of claim 11 further comprising the steps of: generating at said system controller a next third list identifying a next third set of base stations;   providing to said mobile station said next third list;   comparing said next third list to said third list and identifying a third particular base station listing included only on said third list; and   placing said third particular base station listing in said fourth list if a signal strength measurement of a third particular base station corresponding to said third particular base station listing is less than a second predetermined level for a predetermined time interval and said first list contains more than a predetermined number of base station listings.   
     
     
       18. The method of claim 11 further comprising at said mobile station the step of, for each base station having a corresponding signal strength measurement less than a second predetermined level for at least a predetermined time interval, if said first list contains more than a predetermined number of base station listings, removing each corresponding base station listing from said second list and placing each such base station listing in said fourth list. 
     
     
       19. The method of claim 11 further comprising the steps of: providing to said mobile station an update message containing an update list identifying a set of base stations; and   removing a third particular base station listing from said fourth list corresponding to a third particular base station identified by said update list and placing said third particular base station listing on said first list.   
     
     
       20. The method of claim 6 wherein the step of providing by said mobile station said second list to said system controller comprises the steps of: transmitting a message containing said second list by said mobile station to least one of said base stations; and   communicating by said at least one of said base stations to said system controller said second list.   
     
     
       21. The method of claim 11 further comprising the steps of: removing a third particular base station listing from said first list when said first list contains more than a predetermined number of base station listings wherein an age parameter of said third particular base station listing equals or exceeds an age parameter of all other base station listing in said first list; and   placing said third particular base station listing in said fourth list.   
     
     
       22. The method of claim 11 wherein said step of measuring at said mobile station signal strengths of said fourth set of base stations is executed periodically upon execution of a fixed number of executions of said step of measuring at said mobile station signal strengths of said first set of base stations. 
     
     
       23. In a cellular communication system in which a system controller directs communications between a mobile station and at least one base station, a method for mobile station assisted communications directing, comprising the steps of: storing in said mobile station an active list of base stations entries, wherein each entry in said active list corresponds to a base station in communication with said mobile station;   storing in said mobile station a candidate list of base stations entries, wherein each entry in said candidate list corresponds to a base station having sufficient signal strength to establish communication with said mobile station;   transmitting from each base station of said current system a signal identifying said base station;   measuring at said mobile station a signal strength of said corresponding base station transmitted signal for each of said entries in said active list;   measuring at said mobile station a signal strength of said corresponding base station transmitted signal for each of said entries in said candidate list;   comparing said measurement of each of said candidate list entries to each of said measurements of said active list entries; and   identifying by said mobile station to said system controller a particular base station having an entry on said candidate list and another base station having an entry on said active list, if a measurement of said particular base station exceeds a measurement of said another base station by a predetermined amount, T --  COMP.   
     
     
       24. A method of claim 23 further comprising the steps of: advising said particular base station by said system controller to prepare for communication with said mobile station;   generating at said system controller an updated active list containing said entry corresponding to said particular base station;   communicating said updated active list to said mobile station from said system controller; and   establishing communication between said mobile station and said particular base station.   
     
     
       25. A method of claim 23 further comprising the steps of: advising said another base station by said system controller to terminate communication with said mobile station;   generating at said system controller an updated active list excluding said entry corresponding to said particular base station;   communicating said updated active list to said mobile station from said system controller; and   terminating communication by said mobile station through said another base station.   
     
     
       26. A method of claim 25 further comprising the steps of: storing in said mobile station a neighbor list of base stations entries, wherein each entry in said neighbor list corresponds to a base station in a predetermined proximity of said mobile station;   storing in said mobile station a remaining list of base stations entries, each of said base station entries corresponding to a base station of a current system;   comparing said base station signal strength measurements of entry corresponding to said another base station to a first predetermined level, T --  DROP;   placing by said mobile station said entry corresponding to said another base station in said candidate list if one of said measurement of said another base station has exceeded said first predetermined level, T --  DROP, within a first predetermined time interval, T --  TDROP;   placing by said mobile station said entry corresponding to said another base station in said neighbor list if each of said measurements said another base station has been less than said first predetermined level, T --  DROP, for at least said first predetermined time interval, T --  TDROP;   placing by said mobile station said entry corresponding to said another base station in said remaining list if each of said measurements of said first base station has been less than said first predetermined level, T --  DROP, for at least said first predetermined time interval, T --  TDROP, and said neighbor list contains more than a predetermined number of entries.   
     
     
       27. A method of claim 23 further comprising the steps of: comparing at said mobile station said base station signal strength measurements of each of said candidate list entries to a second predetermined level, T --  ADD;   identifying by said mobile station to said system controller a second particular base station having an entry on said candidate list if said measurement of said second particular base station exceeds said second predetermined level, T --  ADD;   advising said second particular base station by said system controller to prepare for communication with said mobile station;   generating at said system controller an updated active list containing said entry corresponding to said second particular base station;   communicating said updated active list to said mobile station from said system controller; and   establishing communication between said mobile station and said second particular base station.   
     
     
       28. A method of claim 23 further comprising the steps of: storing in said mobile station a neighbor list of base stations entries, wherein each entry in said neighbor list corresponds to a base station in a predetermined proximity of said mobile station;   measuring at said mobile station a signal strength of said corresponding base station transmitted signal for each of said entries in said neighbor list;   comparing at said mobile station said base station signal strength measurements of each of said neighbor list entries to a second predetermined level, T --  ADD;   identifying by said mobile station to said system controller a second particular base station having a second particular entry on said neighbor list if said measurement of said second particular base station exceeds said second predetermined level, T --  ADD;   removing by said mobile station said second particular entry from said neighbor list and placing said second particular entry in said candidate list;   advising said second particular base station by said system controller to prepare for communication with said mobile station;   generating at said system controller an updated active list comprised of said second particular entry;   communicating said updated active list to said mobile station from said system controller; and   establishing communication between said mobile station and said second particular base station.   
     
     
       29. A method of claim 23 further comprising the steps of: comparing at said mobile station said base station signal strength measurement of a second particular base station having a second particular entry in said candidate list to a first predetermined level, T --  DROP;   removing by said mobile station said second particular entry from said candidate list and placing said second particular entry in said neighbor list if said measurement of said second particular base station has been less than said first predetermined level, T --  DROP, for at least a first predetermined time interval, T --  TDROP.   
     
     
       30. A method of claim 23 further comprising the steps of: comparing at said mobile station said base station signal strength measurements of a second particular base station having a second particular entry in said active list to a first predetermined level, T --  DROP;   identifying by said mobile station to said system controller said second particular entry if said measurement is less than said first predetermined level, T --  DROP, for at least a first predetermined time interval, T --  TDROP;   advising said second particular base station by said system controller to cease communicating with said mobile station;   generating at said system controller an updated active list excluding said second particular entry;   communicating said updated active list to said mobile station from said system controller; and   terminating communication by said mobile station through said second particular base station.   
     
     
       31. The method of claim 23 further comprising the steps of: storing in said mobile station a neighbor list of base stations entries, wherein each entry in said neighbor list corresponds to a base station in a predetermined proximity of said mobile station;   storing in said mobile station a remaining list of base stations entries, each of said base station entries corresponding to a base station of a current system;   communicating an updated neighbor list to said mobile station from said system controller containing an second particular entry corresponding to a second particular base station; and   removing by said mobile station said second particular entry from said remaining list and placing said second particular entry in said neighbor list.   
     
     
       32. The method of claim 31 further comprising the steps of: storing in said mobile station a neighbor list of base stations entries, wherein each entry in said neighbor list corresponds to a base station in a predetermined proximity of said mobile station;   storing in said mobile station a remaining list of base stations entries, each of said base station entries corresponding to a base station of a current system;   associating at said mobile station with each of said entries in said neighbor list an age parameter;   incrementing each of said associated age parameters upon communication of an updated neighbor list at said mobile station from said system controller; and   removing by said mobile station a third particular entry from said neighbor list and placing said third particular entry in said remaining list if said associated age parameter of said third particular entry exceeds a predetermined age limit.   
     
     
       33. The method of claim 23 further comprising the steps of: storing in said mobile station a neighbor list of base stations entries, wherein each entry in said neighbor list corresponds to a base station in a predetermined proximity of said mobile station;   storing in said mobile station a remaining list of base stations entries, each of said base station entries corresponding to a base station of a current system;   measuring at said mobile station a signal strength of said corresponding base station transmitted signal for each of said entries in said remaining list;   comparing at said mobile station said base station signal strength measurement of each of said remaining list entries to a second predetermined level, T --  ADD;   identifying by said mobile station to said system controller a second particular base station having a second particular entry on said remaining list if said measurement of said second particular base station exceeds said second predetermined level, T --  ADD;   removing by said mobile station said second particular entry from said neighbor list and placing said second particular entry in said candidate list;   advising said second particular base station by said system controller to prepare for communication with said mobile station;   generating at said system controller an updated active list containing second particular entry;   communicating said updated active list to said mobile station from said system controller; and   establishing communication between said mobile station and said second particular base station.   
     
     
       34. In a cellular communication system in which a system controller directs communications between a mobile station and at least one base station, a method for mobile station assisted communications directing, comprising the steps of: identifying by said system controller to said mobile station a first set of base stations which are in a predetermined proximity of said mobile station;   measuring at said mobile station a signal strength of a signal transmitted by each base station in said first set of base stations;   identifying to said system controller by said mobile station a second set of base stations from said first set of base stations having said measured signal strengths sufficient at said mobile station to establish communications therewith;   identifying by said system controller to said mobile station a third set of base stations corresponding to at least one base station of said second set having available resources to establish communication with said mobile station;   notifying by said system controller each base station of said third set of base stations not currently in communication with said mobile station to establish communication with said mobile station; and   establishing communications by said mobile station and each base station of said third set of base stations not currently in communication therewith.   
     
     
       35. A method of claim 34 further comprising the step of terminating existing communication between said mobile station and each base station absent from said third set of base stations. 
     
     
       36. A method of claim 34 further comprising the steps of: measuring at said mobile station a signal strength of a signal transmitted by each base station of said third set of base stations;   identifying to said system controller by said mobile station each base station of said third set of base stations having a measured signal strength which has fallen below a first predetermined level for at least a first predetermined time interval;   notifying by said system controller each said identified base station of said third set of base stations to cease communication with said mobile station;   identifying by said system controller to said mobile station a new third set of base stations wherein each said identified base station of said third set of base stations is absent from said identified new third set of base stations; and   ceasing communications by said mobile station with base stations in said third set of base stations absent from said new third set of base stations.   
     
     
       37. In a cellular communication system in which a system controller controls a handoff in communications between a mobile station and a plurality of base stations, a method for mobile station assisted handoff in said communications comprising the steps of: identifying by said system controller to said mobile station a first set of base stations which are in a predetermined proximity of said mobile station;   measuring at said mobile station a signal strength of a signal transmitted by each base station in said first set of base stations;   determining at said mobile station from said signal strength measurements a second set of base stations, wherein said signal strength measurement of each base station of said second set of base stations is indicative of signal level sufficient to establish communication with said mobile station;   identifying by said mobile station to said system controller said second set of base stations;   determining at said system controller from said second set of base stations a third set of base stations having resources available to establish communication with said mobile station;   notifying by said system controller each base station of said third set of base stations and said mobile station to establish communications between each base station of said third set of base stations if communications therebetween are not currently present;   establishing communication between each base station of said third set of base stations and said mobile station if communications therebetween are not currently present; and   terminating existing communication between said mobile station and each base station absent from said third set of base stations.